This invention relates to radiation-responsive apparatus.
The invention is more particularly, but not exclusively, concerned with radiation pyrometers, such as for use in gas-turbine engines.
The temperature of the blades of a gas-turbine engine can be measured by means of a radiation pyrometer which is mounted to view the blades through an aperture in the engine casing. Such pyrometers have a lens, at one end of a tube, which focusses radiation from an individual blade, as it passes within the field of view of the pyrometer, onto a radiation sensor--either directly, or via a radiation guide, such as a fibre-optic cable. The lens is usually made of a heat-resistant material, such as sapphire, or is mounted behind a window of heat-resistant material, so as to enable it to withstand the high temperatures and temperature changes to which the engine is subject.
One problem with such pyrometers is that soot or other combustion products from the engine can build up on the exposed surface of the lens, thereby reducing the amount of radiation transmitted. Various different arrangements have been proposed to reduce this, such as by the use of a catalyst on the lens surface (GB 2121978A) or by the use of purging air (GB 1589531).
Where purge air is used, the pyrometer is provided with an open sighting tube forwardly of the lens. The pyrometer is arranged to extend across the by-pass duct of the engine, with the forward end of the sighting tube opening into the engine turbine chamber. The sighting tube is provided with air-inlet apertures so that purge air from the by-pass duct, or a separate purge source, enters the sighting tube and flows out the forward end of the tube into the turbine chamber which is at a lower pressure. The flow of clean air through the sighting tube acts to reduce the amount of contaminants entering the pyrometer from the turbine chamber. Although this arrangement does reduce contamination, it is not entirely effective because the turbulent nature of the purge air flow can allow entry of some contaminants from the forward end of the sighting tube.